The present invention is directed to a superconductive composite member which is composed of an oxide ceramic superconductive material being surrounded in an outer envelope consisting essentially of a silver alloy hardened by an oxide dispersion in the alloy and to a method of forming the composite member.
Superconductive composite members wherein an oxide ceramic superconductive powder is surrounded by an enveloping material are known, and examples are disclosed in an article by H. Krauth and A. Szulczyk in METALL, Vol. 43, No. 5, (1989), page 418ff. Composite members, such as wires or bands that are also superconductive above the temperature of liquid nitrogen are manufactured by employing an oxide ceramic high-temperature superconductive material (HTSC). The materials suitable for this purpose are known and include among them, for example, phases selected from the systems of YBaCuO, BiSrCaCuO and TIBaCaCuO.
For manufacturing technical conductors, these superconductive powders are filled, for example, into a metal tube. The wire or band is subsequently manufactured by deforming this tube. As a final step, a thermal treatment is implemented that serves the purpose of producing a continuously superconductive connection on the basis of the high-temperature superconductive material and the purpose of optimizing the critical current density. Since the superconductive properties of the oxidic high-temperature superconductors are greatly influenced by the oxygen content, setting the optimum oxygen content should advantageously occur during the final thermal treatment. This means that the oxygen permeation through the enveloping material must be guaranteed. Silver has proven a satisfactory enveloping material. Silver, however, has the disadvantage that it is extremely soft in mechanical terms. Due to the harder core of the oxide ceramic superconductor material, this extremely soft condition will easily lead to a non-uniform deformation of the core during manufacturing the of the composite member. Moreover, silver has a low mechanical strength after the annealings, which anneatings are needed for producing the optimum superconducting properties of the core.
PCT Application WO 88/08618 proposes that a further, outer layer of steel be applied to the composite member. The steel layer, however, decreases the oxygen permeation to such an extent that the composite member was no longer superconductive. It was, therefore, also proposed in this published application to apply a nitrate coat to the silver envelope or to select a sandwich structure having various combinations of material. The manufacture of such a superconductor, however, is complicated and work-intensive, due to the additional required steps for applying the additional enveloping layers.
European Published Application 290 331 discloses that silver alloys can also be employed as enveloping materials instead of silver. AgCu alloys having a copper content between 2.8% and 30% were disclosed in this publication.
Silver alloys, whose melting point lies above the melting point of pure silver, are disclosed in German Published Application 37 31 266 for being used as enveloping materials for superconductive composite members. These materials were intended to enable the implementation of the thermal treatment at temperatures that lie above the melting point of pure silver. Alloys wherein at least one of the elements selected from a group consisting of gold, palladium, platinum, manganese and titanium are alloyed to the silver and are cited as suitable silver alloys.